Magnesium alloy

ABSTRACT

A refractory magnesium alloy includes magnesium as a principal ingredient, and an element having a radius 9-14% larger than a magnesium atom and a maximum concentration of 2 mass % or larger in a solid solution with magnesium is mixed in an amount not exceeding a maximum amount that can be homogeneously mixed in the solid solution with magnesium, whereby internal strength of grains thereof is enhanced. Alternatively, gadolinium with a content thereof ranging from 0.5 to 3.8 mass % is added, so that remaining part other than the gadolinium is composed of the magnesium and unavoidable impurities. This magnesium alloy serves to inhibit decrease in proof stress and creep deformation, especially primary creep deformation when used at high temperatures, typically at 200° C. The magnesium alloy may be employed for a structural material for a vehicle, so that a lightweight and heat-resistant structural material can be obtained.

TECHNICAL FIELD

[0001] This invention relates to magnesium alloys. More particularly,this invention relates to a magnesium alloy having high heat resistanceand high creep strength that lends itself to structural materials usedunder high temperature conditions.

BACKGROUND ART

[0002] In recent years, out of consideration for the terrestrialenvironment, magnesium alloys (hereinafter referred to as “magnesiummaterials”) are employed for reinforcements making up an engine, aframe, etc. of a vehicle, for purposes of enhancing fuel efficiency ofthe vehicle, for example.

[0003] The magnesium materials have been attracting attention asmaterials having a number of practically excellent properties inapplication for structural materials. To be more specific, magnesium isa metal material that is practically lightest in weight (e.g., specificgravity thereof is approximately two thirds of aluminum andapproximately one fourth of iron), stronger and stiffer than iron andaluminum, having highest capability in absorbing vibrations (dampingcapacity) among practical metal materials, highly resistant to dint,less likely to undergo change in dimension with time or according tovariation of temperature, and easily recyclable. For that reason, themagnesium material is suitable, in particular, for a structural materialfor vehicles, and for a housing of portable terminals.

[0004] However, in cases where a magnesium material is employed for astructural material of a vehicle or the like to be used in ahigh-temperature atmosphere, especially, when the magnesium material isemployed for members making up an engine, which could be exposed to ahigh temperature approximating 200° C., an axial (tightening) force of abolt, for example, would disadvantageously decrease in a portion wheremembers are fastened with the bolt.

[0005] Decrease in axial force of the bolt in such a bolt-fastenedportion may take place due to deformation of a fastened surface of themember or a nut, and it has been conceived that the decrease in axialforce of the bolt would particularly depend upon the creep strength ofthe material.

[0006] As a result, in order to prevent the decrease in axial force ofthe bolt, various kinds of magnesium materials (magnesium alloys) havingimproved creep strength have been developed.

[0007] For example, a refractory magnesium alloy containing aluminum,zinc, or the like each in a specific proportion and formed by addingsilica, rare-earth metal, calcium and the like is known in the art.

[0008] However, with a conventional magnesium alloy as above, thoughsome improvement in creep strength can be achieved, it is difficult tocheck the decrease in axial force of the bolt on the condition that amember made up of the magnesium alloy is used in a relativelyhigh-temperature atmosphere, and thus the aforementioned disadvantagescannot be deemed to be finally rooted up.

DISCLOSURE OF INVENTION

[0009] It is an object of the present invention to provide a magnesiumalloy having high thermal resistance with which an axial force of a boltdoes not decrease even when used under high temperature conditions,preferably at 150° C., and more preferably at 200° C., and a magnesiumalloy having high creep strength which can prevent an axial force of abolt from decreasing when a member such as of a structural materialprovided around an engine of a vehicle, or the like is used under hightemperature conditions, preferably at 150° C., and more preferably at200° C.

BRIEF DESCRIPTION OF DRAWINGS

[0010]FIG. 1 is a schematic diagram for explaining a member fastenedwith a bolt which member is made of a magnesium material.

[0011]FIG. 2 is a graph showing a relationship between axial forces of abolt and time, with the bolt-fastened member made of magnesium materialexposed under high temperature conditions.

[0012]FIG. 3 is a graph showing a relationship between creep elongationand time with magnesium exposed under high temperature conditions.

[0013]FIG. 4 is a diagram showing amounts of increase in hardness ofalloys with respect to a difference between atomic radii of firstcomponents mixed with magnesium to be a solid solution, and the atomicradius of magnesium.

[0014]FIG. 5 is a diagram showing a relationship between maximum amountsof the first component mixed with magnesium to be a solid solution, andamounts of increase in hardness of magnesium alloys.

[0015]FIG. 6 is a diagram showing a relationship between percentages ofdifferences between atomic radii of the first components homogeneouslymixed with magnesium to be a solid solution and the atomic radius ofmagnesium, and maximum amounts of the first components capable of beinghomogeneously mixed with magnesium into a solid solution.

[0016]FIG. 7 is a diagram showing a relationship between experimentallydetermined eutectic points of a second component mixed with magnesium tobe a solid solution, and steady-state creep elongations.

[0017]FIG. 8 is a diagram showing a relationship between a zirconium(Zr) content in an alloy to which zirconium (Zr) is added as a thirdcomponent, and particle sizes of the alloy.

[0018]FIG. 9 is a graph showing a relationship between the time requiredtill molten metal of magnesium with an yttrium (Y) content startsburning, and amounts of yttrium (Y) added.

MODE(S) FOR CARRYING OUT THE INVENTION

[0019] Magnesium alloys according to the present invention includesfirst through third modes of magnesium alloys as will be describedbelow. A detailed description will hereinafter be given of components ofthe first through third modes of the magnesium alloys according to thepresent invention.

[0020] The first through third modes of the magnesium alloys accordingto the present invention are magnesium-based alloys to whichpredetermined amounts of various components are added.

[0021] <Magnesium>

[0022] Magnesium is a base-metal element of the first through thirdmodes of the present invention. Thus, the features of magnesium employedfor a structural material will now be described. Hereupon, the term“magnesium material” will be used when general properties of magnesiumas a structural material are explained in the following description.

[0023] As describe above, the magnesium material is practically mostlightweight metal, stronger and stiffer than iron and aluminum, havinghighest capability in absorbing vibrations (damping capacity) amongpractical metal materials, highly resistant to dint, less likely toundergo change in dimension even in a high-temperature atmosphere, andeasily recyclable.

[0024] However, when the magnesium material is exposed in ahigh-temperature atmosphere approximating 200° C., the magnesiummaterial creeps, for example a bolt-fastened member made of magnesiummaterial creeps, and an axial force of a bolt decreases.

[0025] A change of the axial force of a bolt when a bolt-fastened member(FIG. 1) made of a magnesium material is exposed in a high-temperatureatmosphere will now be described with reference to a graph (FIG. 2)showing a relationship between axial forces of the bolt and time.

[0026] When the bolt-fastened member is exposed in a high-temperatureatmosphere, the axial force of the bolt rises, then drops sharply for arelatively short time, and subsequently decreases gradually. Thisphenomenon may presumably be attributable to the following reasons.

[0027] Immediately after the bolt is fastened to the member made ofmagnesium material, a stress applied to the fastened surface of themagnesium material and the nut is not more than a proof stress thereof,and thus the magnesium material has undergone no creep. Accordingly, theaxial force of the bolt is retained at a sufficient level (RANGE a inFIG. 2).

[0028] When the bolt-fastened member made of magnesium material isexposed in a high-temperature atmosphere, the stress applied to thefastened surface of the magnesium material and the nut increases, whilethe strength of the magnesium material decreases due to increase intemperature.

[0029] Immediately after the bolt-fastened member is exposed in ahigh-temperature atmosphere, the “thermal stress” generated due to adifference in thermal expansion amount between the magnesium materialand the bolt material (FIG. 1; steel material) causes the axial force ofthe bolt to increase (RANGE b in FIG. 2).

[0030] Thereafter, when the stress applied to the fastened surface madeof magnesium material and the nut exceeds the strength of the magnesiummaterial, the magnesium material undergoes permanent deformation (creep)and the axial force of the bolt decreases (RANGE c in FIG. 2).

[0031] The decrease in axial force of the bolt goes on till the stressapplied to the magnesium material reaches the proof stress or maximumlevel of strength durable of the magnesium material at that temperature.

[0032] When the stress applied to the magnesium material reaches theproof stress of the magnesium material at that temperature, the axialforce of the bolt appears prevented from sharply decreasing, butthereafter steady-state creep deformation that appears under a lowerstress causes a relatively gentle decrease in axial force of the bolt toprogress (RANGE d in FIG. 2).

[0033] Next, a progress of creep deformation of a magnesium materialwill be described with reference to a graph (FIG. 3) showing arelationship between creep elongation of the magnesium material in ahigh-temperature atmosphere and time.

[0034] When a magnesium material to which a specific level of stress isapplied is exposed in a high-temperature atmosphere, a creep(elongation) appears within a relatively short time (RANGE a in FIG. 3).Such a creep will be hereinafter referred to “primary creepdeformation”. Thereafter, a relatively gentle creep (elongation)progresses, as time goes by (RANGE b in FIG. 3). Such a gentle creepwill be hereinafter referred to as “steady-state creep deformation”.

[0035] Subsequently, a description will be given of a mechanism thatproduces deformation as observed in a tensile test or a creep test.

[0036] Metal magnesium is a polycrystal that consists of aggregates ofgrains of magnesium. Grain boundaries exist between individual grains.

[0037] Since deformation is observed in individual grains of magnesiumin the short-time range of the tensile test or creep test, it is assumedthat the proof stress and primary creep deformation are subject to thestrength of the grains.

[0038] On the other hand, since deformation is observed in grainboundaries of magnesium, and besides very small holes generate, in thelong-time range of the creep test, it is assumed that the steady-statecreep deformation is subject to the strength of the grain boundaries.

[0039] In the grains, there are atoms of magnesium that are regularlyand three-dimensionally arranged. Such regular arrangement of magnesiumatoms may easily be deformed by external forces. In principle, thedeformation is mainly caused by dislocation of the atoms.

[0040] On the other hand, grain boundaries are portions that are formedin the last place during a manufacturing (casting) process, and elementsother than magnesium and/or compounds incorporated in components of themetal magnesium are likely to be distributed therein. The grainboundaries, of which arrangement includes magnesium atoms and elementsother than magnesium, suffer lattice defects caused by missing atoms inplaces. Under high temperature conditions, cohesive forces between atomsdecrease due to increasing thermal vibrations. As a result, the atoms inthe grain boundaries become likely to move to neighboring lattice defectportions more frequently. This phenomenon is called diffusion. As thediffusion progresses, grain boundaries deform.

[0041] To sum up the above, the decrease in axial force of a bolt in abolt-fastened member made of magnesium material may be explained asfollows. A sharp decrease in axial force of the bolt shown immediatelyafter the bolt-fastened magnesium member is exposed under hightemperature conditions results from the reduced proof stress and theprimary creep deformation, which is subject to the internal strength ofthe grains, while a subsequent gradual decrease in axial force of thebolt results from the steady-state creep deformation, which is subjectto the strength of the grain boundaries.

[0042] Accordingly, in order to inhibit the decrease in axial force ofthe bolt in the bolt-fastened member made of magnesium material for useunder high temperature conditions, attempts should be made to improvethe proof stress and to impede the primary creep deformation as well asto impede the steady-state creep deformation, because only such anattempt to impede creep deformation, particularly only an attempt toimpede the steady-state creep deformation, as has been made in theconventional efforts to develop refractory alloys, could not prevent theaxial force of the bolt from decreasing.

[0043] To improve the proof stress and impede the primary creepdeformation, deformation in the individual grains of magnesium need bechecked, whereas to impede the steady-state creep deformation, diffusionin the grain boundaries need be inhibited.

[0044] [First Mode]

[0045] A description will be given below of requirements for a firstmode of magnesium alloys according to the present invention. The firstmode of magnesium alloys according to the present invention ischaracterized by having an element (hereinafter referred to as “firstcomponent”) in an amount not exceeding the maximum amount that can behomogeneously mixed in a solid solution with magnesium as describedabove which first component has a radius 9-14% larger than a magnesiumatom and a maximum concentration of 2 mass % or larger in the solidsolution, for the purposes of preventing the decrease of proof stressunder high temperature conditions and the deformation derived from theprimary creep (RANGE a in FIG. 2).

[0046] <First Component: an Element Having a Radius 9-14% Larger Than aMagnesium Atom, and a Maximum Concentration of 2 Mass % or Larger in aSolid Solution Mixed with Magnesium>

[0047] In the first mode of magnesium alloys according to the presentinvention, atoms of the first component homogeneously mixed withmagnesium into a solid solution are partly substituted for atoms ofmagnesium in the grains to form a substitution solid solution, and thusa microscopic lattice distortion appears in crystals. Then, themicroscopic lattice distortion serves to inhibit deformation in grainsof magnesium which would appear when the magnesium is exposed in hightemperature surroundings. As a result, the proof stress or tensilestrength which is subject to the internal strength of the grains can beimproved, and the primary creep deformation can be impeded.

[0048] It is a finding experimentally demonstrated by the presentinventors and their colleagues that if the radius of atoms of the firstcomponent homogeneously mixed with magnesium into a solid solution isout of the range 9-14% larger than that of atoms of magnesium, theeffect of improvement in the proof stress of the magnesium alloy can notsufficiently be achieved.

[0049] This is because of the diminished lattice distortion of asubstitution solid solution obtained when the atomic radius of the firstcomponent to be homogeneously mixed with magnesium into a solid solutionis smaller than the size 9% larger than the atomic radius of magnesium,which would lessen the effect of inhibiting deformation in grains.

[0050] Another reason therefor is as follows. The lattice distortion ofthe substitution solid solution obtained when the atomic radius of thefirst component to be homogeneously mixed into a solid solution exceedsthe size 14% larger than the atomic radius of magnesium is large enough,but the amount of the first component mixed with magnesium would be sosmall that a solid solution could not readily be formed.

[0051] Further, if an element of which the atomic radius is smaller thanthat of magnesium is homogeneously mixed with magnesium into a solidsolution, the intended effect of improvement in the proof stress can notsufficiently be achieved. This is based upon the same reason as in thecase where the atomic radius of the element is smaller than the size 9%larger than that of magnesium as described above.

[0052] In actuality, when an element whose atomic radius is smaller thanthat of magnesium was homogeneously mixed with magnesium into a solidsolution to form an alloy, the strength of the alloy conspicuouslylowered by exposure of heat, and the effect of inhibiting deformation ingrains was limited.

[0053]FIG. 4 shows increase in hardness of an alloy versus percentage ofdifferences between experimentally obtained atomic radius of the firstelement homogeneously mixed with magnesium into a solid solution and theatomic radius of magnesium.

[0054] As apparent from FIG. 4, gadolinium (Gd) of which the atomicradius is 11.3% larger than that of magnesium exhibited an increase of65.5 in hardness; among others, yttrium (Y) of which the atomic radiusis 13.8% larger exhibited an increase of 54.1; neodymium (Nd) of whichthe atomic radius is 13.8% larger exhibited an increase of 39.2; andsamarium (Sm) of which the atomic radius is 11.9% larger exhibited anincrease of 33.8.

[0055] Among common elements of which the atomic radius is smaller thanthe size 9% larger than the atomic radius of magnesium, Bismuth (Bi)having an atomic radius of −3.1% larger exhibited an increase of 23.9 inhardness.

[0056] As described above, if the atomic radius of the first componentfalls within a range 9-14% larger than the atomic radius of magnesium, adesirable increase in hardness is achieved.

[0057] Further, if the maximum amount of the first component that canhomogeneously be mixed with magnesium into a solid solution is notexceeding 2 mass %, the effect of improvement in the proof stress wasnot able to be achieved.

[0058] This is because the maximum amount of the first component thatcan be homogeneously mixed with magnesium into a solid solution is sosmall that a proportion of the atoms of the first component for whichthe atoms of magnesium are substituted (the rate of formation ofsubstitution solid solution) is low.

[0059]FIG. 5 shows a relation between experimentally obtained maximumamounts of the first component homogeneously mixed with magnesium into asolid solution and increase in hardness of the alloys.

[0060] As apparent from FIG. 5, gadolinium (Gd) of which the maximumamount capable of being homogeneously mixed with magnesium atoms into asolid solution is 3.82 mass % exhibited an increase of 65.5 in hardness;among others, yttrium (Y) of which the maximum amount capable of being asolid solution is 2.20 mass % exhibited an increase of 54.1; neodymium(Nd) of which the maximum amount capable of being a solid solution is0.12 mass % exhibited an increase of 39.2; and samarium (Sm) of whichthe maximum amount capable of being a solid solution is 0.39 mass %exhibited an increase of 33.8.

[0061] On the other hand, among other elements out of the scope of thepresent invention, for example, lead (Pb) of which the maximum amountcapable of being a solid solution is 3.31 mass %, aluminum (Al) of whichthe maximum amount capable of being a solid solution is 3.32 mass %exhibited increases of 23.9 and 19.5 respectively; the increasecomparable to the rare-earth elements could not be achieved. Tin (Sn)and Gallium (Ga) of which the maximum amount capable of being a solidsolution is not exceeding 2.0 mass % exhibited the same consequences.

[0062] As clearly shown from the above, if the maximum amount of thefirst component capable of homogeneously mixed with magnesium atoms intoa solid solution is equal to or greater than 2 mass %, a favorableincrease in hardness can be achieved.

[0063] For reasons as described above, preferable first components to behomogeneously mixed with magnesium into a solid solution may includeholmium (Ho), dysprosium (Dy), terbium (Tb), gadolinium (Gd), yttrium(Y), and the like, of which the atomic radius is 9-14% larger than theradius of magnesium atoms, and the maximum amount capable of being asolid solution with magnesium is equal to or greater than 2 mass %.

[0064]FIG. 6 is a diagram showing a relationship between percentages ofdifferences between atomic radii of the first components homogeneouslymixed with magnesium to be a solid solution and the atomic radius ofmagnesium, and maximum amounts of the first components capable of beinghomogeneously mixed with magnesium into a solid solution. Values shownin FIG. 6 are also those obtained by experiment.

[0065] As clearly shown in the above discussion, the use of therare-earth elements of which the atomic radii are 9-14% larger than thatof magnesium and the maximum amounts capable of being a solid solutionwith magnesium atoms are equal to or greater than 2 mass % would lead tofavorable effects.

[0066] To be more specific, other than the aforementioned firstcomponents, for example, lutetium (Lu), erbium (Er), thulium (Tm) andsuch rare-earth elements may be added to produce alloys havingsuperiority in hardness.

[0067] Conventionally designed preparation of magnesium alloys utilizesaluminum (Al) or zinc (Zn) for an additive element in many instances.However, the atomic radii thereof are smaller than that of magnesium,and thus the use thereof could not produce alloys exhibiting preferredhardness in high temperature surroundings.

[0068] Accordingly, mixing such elements as are out of theaforementioned range (i.e., the atomic radius thereof is 9-14% largerthan the atomic radius of magnesium; the maximum amount capable of beinga solid solution with magnesium is equal to or greater than 2 mass %)with magnesium is not preferable to form the present mode of magnesiumalloys.

[0069] Hereupon, it is understood as described above that an upper limitof the amount of the first component to be added is the maximum amountof the element capable of being a solid solution with magnesium as thefirst component is to be homogeneously mixed with magnesium into a solidsolution. A lower limit of the amount is not particularly restricted, asfar as the amount is sufficient to achieve the objects of the presentinvention. Therefore, the amount of the first component to be added maybe determined as appropriate with consideration given to costs and thelike of magnesium alloys to be produced.

[0070] To illustrate more specifically, for example, assuming thatgadolinium (Ga) is chosen as an element to be added, the amount thereofto be added is preferably 0.5-3.8 mass %, more preferably 1.0-3.5 mass%, or so, as will be described later.

[0071] <Second Component>

[0072] In the first mode of magnesium alloys according to the presentinvention, in addition to the above-described first component, anelement of which a mixture with magnesium has an eutectic point of 540°C. or greater (hereinafter referred to as “second component”) mayfurther be added, as will be described later, for the purpose ofimpeding steady-state creep.

[0073] Usable elements for the second components are elements of which amixture with magnesium has a eutectic point of 540° C. or greater and amelting point lower than magnesium. Preferably, for example, lanthanum(La), cerium (Ce), neodymium (Nd), or other rare-earth elements, or tin(Sn), barium (Ba), etc. may be added.

[0074] To be more specific, when the second component that meets theabove conditions is added, the second component forms eutectic compoundswith atoms of magnesium, and the eutectic compounds diffuse intointerfaces or grain boundaries between individual grains making upmagnesium. Since the eutectic compounds formed as above are stable athigh temperatures, diffusion of atoms in grain boundaries can beeffectively inhibited even under high temperature conditions, and thusthe steady-state creep of magnesium alloys can be impeded.

[0075] Hereupon, if the eutectic point of the mixture of the secondcomponent with magnesium were lower than 540° C., the steady-state creepelongation would disadvantageously become greater. This is becausediffusion of atoms becomes likely to occur in the eutectic compoundunder high temperature conditions and thus deformation in the grainboundaries cannot be impeded at high temperatures.

[0076]FIG. 7 shows a relationship between experimentally determinedeutectic points of the second component mixed with magnesium to be asolid solution, and steady-state creep elongations thereof.

[0077] As apparently shown in FIG. 7, gadolinium (Gd), cerium (Ce) orthe like of which a mixture with magnesium has an eutectic point of 540°C. or greater exhibits a minimized steady-state creep elongation (%).

[0078] Consequently, the second component used for the present inventionis preferably an element of which a mixture with magnesium has aeutectic point of 540° C. or greater and which has a melting point lowerthan magnesium.

[0079] The effect of impeding steady-state creep deformation is subjectto a temperature at which the compound is formed, i.e., the eutecticpoint; the effect is enhanced in accordance with the temperature.Accordingly, elements among lanthanoids as recited above may be arrangedin descending order of effectiveness as follows: lanthanum (La), cerium(Ce), praseodymium (Pr), europium (Eu), neodymium (Nd), and samarium(Sm).

[0080] With respect to the amount of the second component to be added,if the rate of the second component added were less than one mass %, theamount of the eutectic compounds generated would be reduced, and thusdiffusion of atoms occurring in the grain boundaries could not beinhibited, so that the objects and advantages expected from addition ofthe second component could not sufficiently be achieved. If the rate ofthe second component added were 15 mass % or greater, the amount of theeutectic compounds generated would become too much, and thus elongationcapability of the magnesium alloys would disadvantageously be lowered toan appreciable extent.

[0081] The magnesium material applied to a structural material needs tohave a sufficient strength at high temperatures, i.e., tensile strength,proof stress, and creep strength, but such an arrangement as preparedwith consideration given only to the strength at high temperatures wouldinvolve a difficulty in practicability in some instances. It is abalance kept between strength and elongation that matters. It isunderstood as an adequate level of elongation that the structuralmaterial, in particular as used in an engine for a vehicle, needs tohave an elongation percentage of approximately 2.0% or greater.Therefore, the sufficient strength at high temperatures and sufficientlevel of elongation should both be secured.

[0082] Consequently, the amount of the second component to be addedaccording to the present invention falls within the range of: preferably1-15 mass %, and more preferably 3-8 mass %.

[0083] <Third Component>

[0084] Moreover, the first mode of magnesium alloys according to thepresent invention may further contain in addition to the above firstcomponent and second component one or more elements selected from agroup consisting of zirconium (Zr), strontium (Sr), and manganese (Mn)(hereinafter referred to as “third component”) with a content thereofbeing less than 1 mass %.

[0085] Minor amounts of the elements as recited above are added to themagnesium alloys, to make grain sizes of magnesium crystals smaller.

[0086] The grain size of each crystal of magnesium alloys greatlydepends upon solidification rates in general, and the smaller the grainsizes of the crystals, the greater the proof stress tends to be.

[0087] In a thick portion, the solidification progresses slowly;resultantly the grain sizes of the crystals tend to become larger andthe strength thereof tends to become lower.

[0088] With the third component, even in thick portions in which thesolidification progresses slowly, the grain sizes of the crystals can bemade very small, as small sizes as can be attained in thinner portionsin which the solidification progresses more rapidly. Moreover, thecompounds in the grain boundaries diffuse evenly, and variation ofstrength under high temperature conditions in each portion can therebyfall within an adequately narrow range.

[0089] A relationship between a zirconium (Zr) content in an alloy, towhich zirconium (Zr) is added as the third component, and grain sizes ofthe alloy is shown in FIG. 8. In other words, the graph shows variationof grain sizes in accordance with the amount of zirconium (Zr) added tothe magnesium alloy according to the present invention, which amountranges from 0.0 through 1.2 mass %.

[0090] As shown in FIG. 8, as the amount of zirconium (Zr) addedincreases, the grain size of crystals decreases. When zirconium (Zr)exceeding 0.8 mass % is added, the effect of addition of zirconium (Zr)shows up at a maximum thereof. Since zirconium (Zr) reacts withmagnesium to form peritectoid, and upon solidification, zirconium (Zr)becomes a solidification nucleus of a magnesium crystal, the grainbecomes small.

[0091] When the amount of the third component added becomes 1 mass % orgreater, a great number of relatively brittle compounds are generated inthe grains or grain boundaries. The relatively brittle compounds couldcause brittle fracture; therefore, a great number of the relativelybrittle compounds generated in the grains or grain boundaries wouldmarkedly lower the elongation capability of the magnesium alloys, andlower the strength of the magnesium alloys.

[0092] It is understood that the effect of addition of the thirdcomponent can be achieved when strontium (Sr) or manganese (Mn) is used,as well.

[0093] Accordingly, the amount of addition of the third component in thefirst mode of magnesium alloys according to the present inventionpreferably falls below 1 mass %, and more preferably ranges between 0.5and 0.8 mass %.

[0094] The third component does not necessarily have to be used incombination with the first component and the second component, but maybe used only with the first component.

[0095] In this instance, the magnesium alloys composed of magnesium, thefirst component and the third component can serve to effectively achieveimprovement of the proof stress and impede the primary creep deformationunder high temperature environments through formation of thesubstitution solid solution and small-sized grains.

[0096] As discussed above, the first mode of magnesium alloys accordingto the present invention exhibits high proof stress and high creepstrength under high temperature conditions, and can thus be employed forstructural materials to be used under high temperature conditions, suchas structural materials for a vehicle, in particular, those which lenditself to a cylinder block, a cylinder head, an intake manifold, a headcover, a chain case, an oil pan, a transmission case, an ECU frame, andother structural members to be mounted around the engine of the vehicle.

[0097] [Second Mode]

[0098] A description will be given below of requirements for a secondmode of magnesium alloys according to the present invention. The secondmode of magnesium alloys is characterized by having a gadolinium (Gd)content of 0.5-3.8 mass % in above-described magnesium as a principalingredient, i.e., the remaining part is composed of magnesium andunavoidable impurities.

[0099] <First component: Gd>

[0100] The second mode of magnesium alloys has a gadolinium, as a firstcomponent, in an amount of 0.5-3.8 mass %, homogeneously mixed in asolid solution with magnesium for the purposes of preventing thedecrease of proof stress under high temperature conditions and thedeformation derived from the primary creep (RANGE a in FIG. 3).

[0101] In the second mode of magnesium alloys of this composition, atomsof the first component homogeneously mixed into a solid solution aresubstituted for some of magnesium atoms in the grains to form asubstitution solid solution, and microscopic lattice distortion is thusgenerated in the crystals. Then the microscopic lattice distortionserves to inhibit deformation in grains of magnesium which would appearwhen the magnesium is exposed in high temperature surroundings. As aresult, the proof stress and tensile strength which are subject to theinternal strength of the grains can be improved, and the primary creepdeformation can be impeded.

[0102] The reason why gadolinium is selected as the first component ofthe second mode of magnesium alloys is that the atomic radius ofgadolinium is larger than that of magnesium, and that the maximum amountof gadolinium allowed to be a solid solution when mixed with magnesiumis larger, and thus the effect of inhibiting deformation is higher, thanany other elements.

[0103] Since gadolinium is to be a solid solution when mixed withmagnesium, even if more than the maximum amount of gadolinium allowed toform a solid solution in magnesium is added, an excess amount ofgadolinium is not homogeneously mixed with magnesium into a solidsolution. Therefore, in the present invention, the upper limit of agadolinium content is 3.8 mass % that is the maximum amount ofgadolinium allowed to form a solid solution.

[0104] The lower limit of a gadolinium content to be mixed into a solidsolution is not restricted to a specific value, as far as the object ofthe present invention can be achieved. The amount may be determined asappropriate with consideration given to manufacturing costs of magnesiumalloys, or the like.

[0105] Consequently, the gadolinium content in the second mode ofmagnesium alloys according to the present invention is preferably0.5-3.8 mass %, and more preferably 1.0-3.5 mass %, or so.

[0106] <Second Component>

[0107] The second mode of magnesium alloys according to the presentinvention may further include in addition to gadolinium as the abovefirst component, one or more elements selected from a group consistingof lanthanum through europium among lanthanoids in the periodic table ofthe elements (hereinafter referred to as “second component”) with acontent thereof ranging from 1 to 15 mass % for the purpose of impedingsteady-state creep deformation.

[0108] The second components that may preferably be added includelanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd),promethium (Pm), samarium (Sm), and so forth.

[0109] To be more specific, when the second component that meets theabove conditions is added, the second component is combined with atomsof magnesium to form eutectic compounds, and the eutectic compoundsdiffuse into grain boundaries. Since thus-formed eutectic compounds arestable at high temperatures, diffusion of atoms in grain boundaries canbe effectively inhibited even under high temperature conditions, andthus the steady-state creep of magnesium alloys can be impeded.

[0110] The effect of impeding the steady-state creep deformationincreases with the temperature at which the compounds are formed, i.e.,the eutectic point; the effect is enhanced in accordance with thetemperature. Among the aforementioned lanthanoids, the effect is likelyto become higher in the following order: lanthanum, cerium,praseodymium, europium, neodymium, and samarium.

[0111] In particular, since the maximum amount of the elements belongingto the group consisting of lanthanoids as exemplified above that may behomogeneously mixed with magnesium atoms into a solid solution is small,even if a small amount of the elements are added, eutectic compounds areformed in the grain boundaries.

[0112] The effect of impeding steady-state creep deformation is subjectto a temperature at which the compound is formed, i.e., the eutecticpoint; the higher the eutectic point, the higher the effect becomes;accordingly, elements among lanthanoids as recited above may be arrangedin descending order of effectiveness as follows: lanthanum, cerium,praseodymium, europium, neodymium, and samarium.

[0113] Hereupon, if the rate of the second component added were lessthan one mass %, the amount of the eutectic compounds formed would bereduced, and thus diffusion of atoms occurring in the grain boundariescould not be inhibited, so that the objects and advantages expected fromaddition of the second component could not sufficiently be achieved.

[0114] Moreover, if the rate of the second component added were 15 mass% or greater, the amount of the eutectic compounds formed would becometoo much, and thus elongation capability of the magnesium alloys woulddisadvantageously be lowered to an appreciable extent.

[0115] The magnesium material applied to a structural material needs tohave a sufficient strength at high temperatures, i.e., tensile strength,proof stress, and creep strength, but such an arrangement as preparedwith consideration given only to the strength at high temperatures wouldinvolve a difficulty in practicability in some instances. It is abalance kept between strength and elongation that matters. It isunderstood as an adequate level of elongation that the structuralmaterial, in particular as used in an engine for a vehicle, needs tohave an elongation percentage of approximately 2.0% or greater.Therefore, the sufficient strength at high temperatures and sufficientlevel of elongation should both be secured.

[0116] Accordingly, the amount of the second component to be addedaccording to the present invention falls within the range of: preferably1-15 mass %, and more preferably 3-8 mass %.

[0117] Consequently, the second mode of magnesium alloys which iscomposed of magnesium, the first component and the second componentbrings about inhibition of steady-state creep deformation due toeutectic compounds, as well as improvement of proof stress andinhibition of primary creep deformation due to formation of theabove-described substitution solid solution, and can thus achieveeffective improvement of the proof stress and creep strength ofmagnesium alloys under high temperature environments.

[0118] <Third Component>

[0119] Moreover, the second mode of magnesium alloys as described abovemay further contain in addition to the above first component and secondcomponent as in the first mode one or more elements selected from agroup consisting of zirconium, strontium, and manganese (hereinafterreferred to as “third component”) with a content thereof being less than1 mass %, as in the first mode.

[0120] The functions performed by these elements are the same asdiscussed in the first mode, and thus a description thereof will beomitted herein.

[0121] With the third component, even in portions where thesolidification progresses slowly, the grain sizes of the crystals can bemade very small, as small sizes as can be attained in thinner portionswhere the solidification progresses faster. Moreover, the compounds inthe grain boundaries diffuse evenly, and variation of strength underhigh temperature conditions in each portion can thereby fall within anadequately narrow range.

[0122] A relationship between a Zr content in an alloy, to which Zr isadded as the third component, and grain sizes of the alloy is shown inFIG. 8. In other words, FIG. 8 shows variation of grain sizes inaccordance with the amount of Zr added to the magnesium alloy accordingto the present invention, which amount ranges from 0.0 through 1.2 mass%. As shown in FIG. 8, as the amount of Zr added increases, the grainsize of crystals decreases. When Zr of which the content exceeds 0.8mass % is added, the effect of addition of Zr shows up at a maximumthereof.

[0123] Hereupon, when the amount of the third component added becomes 1mass % or greater, a great number of relatively brittle compounds aregenerated in the grains or grain boundaries. Therefore, the relativelybrittle compounds could cause the elongation capability of the magnesiumalloys to decrease drastically, and the strength of the magnesium alloysto decline.

[0124] Accordingly, the amount of addition of the third component in thesecond mode of magnesium alloys according to the present inventionpreferably falls below 1 mass %, and more preferably ranges between 0.5and 0.8 mass %.

[0125] The third component does not necessarily have to be used incombination with the first component and the second component, but maybe used only with the first component.

[0126] In this instance, the magnesium alloys composed of magnesium, thefirst component and the third component can serve to effectively achieveimprovement of the proof stress and impede the primary creep deformationunder high temperature environments through formation of thesubstitution solid solution and small-sized grains.

[0127] As discussed above, the second mode of magnesium alloys accordingto the present invention exhibits high proof stress and high creepstrength under high temperature conditions, and can thus be employed forstructural materials to be used under high temperature conditions, suchas structural materials for a vehicle, in particular, those which lenditself to a cylinder block, a cylinder head, an intake manifold, a headcover, a chain case, an oil pan, a transmission case, an ECU frame, andother structural members to be mounted around the engine of the vehicle.

[0128] [Third Mode]

[0129] A description will be given below of requirements for a thirdmode of magnesium alloys according to the present invention. The thirdmode of magnesium alloys is characterized by having a cerium content of2.0-10.0 mass %, a tin content of 1.4-7.0 mass % in magnesium as aprincipal ingredient, i.e., the remaining part is composed of magnesiumand unavoidable impurities.

[0130] <First Component: Ce+Sn>

[0131] The third mode of magnesium alloys contains cerium (Ce) in anamount of 2.010.0 mass %, preferably 4.0-6.0 mass %, and tin (Sn) in anamount of 1.4-7.0 mass %, preferably 3.5-6.5 mass % (hereinafterreferred to as “first component”) in addition to magnesium as aprincipal ingredient for the purpose of impeding steady-state creep.Thus, the third mode of magnesium alloys is designed to impedesteady-state creep in magnesium alloys to be prepared, utilizing asynergistic effect of addition of the both elements, cerium (Ce) and tin(Sn) each added in specific amounts.

[0132] Hereupon, when cerium and tin are added to magnesium, an aciculareutectic compound with the ternary system of magnesium, cerium and tinis formed. Thus-formed eutectic compounds then diffuse into interfacesor grain boundaries between individual grains making up magnesium. As aresult, the eutectic compounds which are stable at high temperaturesserve to inhibit diffusion that could be caused under high temperatureenvironments, and the inhibitive capability inherent in acicularcompounds serves to inhibit a slip of one grain over another, therebyinhibiting creep deformation of the magnesium alloys.

[0133] If the amount of cerium (Ce) added were below 2.0 mass %, or ifthe amount of tin (Sn) added were below 1.4 mass %, the amount ofeutectic compounds that could be formed would be insufficient, and thusthe steady-state creep deformation could not be inhibited sufficiently.

[0134] Conversely, if the amount of cerium (Ce) added were more than10.0 mass %, or if the amount of tin (Sn) added were more than 7.0 mass%, the amount of eutectic compounds that could be formed would becometoo much, and thus the elongation capability of the magnesium alloyswould be lowered to an appreciable extent.

[0135] Accordingly, the amounts of cerium (Ce) and/or tin (Sn) to beadded to magnesium alloys falling out of the above ranges respectivelywould disadvantageously prevent the resulting magnesium alloys fromachieving sufficiently high creep strength.

[0136] The ratio between the amounts of cerium (Ce) and tin (Sn) to beadded ranges preferably 0.6-1.4, and more preferably 1.0-1.2,(cerium-to-tin ratio, Ce/Sn) in mass.

[0137] If the ratio (cerium to tin, Ce/Sn) in mass were less than 0.6,the percentage of Mg—Sn compounds and/or single-phase tin (Sn) thatcould be formed in the grain boundaries in addition to the eutecticcompounds having the ternary system of magnesium, cerium and tin wouldincrease.

[0138] Of the two, particularly, the single-phase tin (Sn) has a lowmelting point, and thus the creep strength of magnesium alloys tends todecrease with increase in percentage of the single-phase tin (Sn)contained in the grain boundaries.

[0139] If the ratio (cerium to tin, Ce/Sn) in mass were more than 1.4,the percentage of Mg—Ce compounds that could be formed in the grainboundaries in addition to the eutectic compounds having the ternarysystem of magnesium, cerium and tin would increase.

[0140] The Mg—Ce compounds are lower in stability than Mg—Ce—Sn ternarycompounds at 150° C. or higher, and thus the creep strength of magnesiumalloys tends to decrease with increase in percentage of the Mg-Cecompounds contained in the grain boundaries.

[0141] Therefore, the ratio between the amounts of cerium (Ce) and tin(Sn) to be added preferably falls within the above range.

[0142] <Second Component>

[0143] Moreover, the third mode of magnesium alloys according to thepresent invention may further contain in addition to the above-describedfirst component one or more elements selected from a group consisting ofzirconium (Zr), strontium (Sr), and manganese (Mn) (hereinafter referredto as “second component”) with a content thereof being less than 1 mass%.

[0144] Minor amounts of the elements as recited above are added to themagnesium alloys, to make grain sizes of magnesium crystals smaller.

[0145] As discussed above, the grain size of each crystal of magnesiumalloys greatly depends upon solidification rates in general, and thesmaller the grain sizes of the crystals, the greater the proof stresstends to be.

[0146] In a thick portion of magnesium alloys, the solidificationprogresses slowly; resultantly the grain sizes of the crystals tend tobecome larger and the strength thereof tends to become lower.

[0147] With the second component, even in thick portions in which thesolidification progresses at a relatively low speed, the grain sizes ofthe crystals can be made very small, as small sizes as can be attainedin thinner portions in which the solidification progresses more rapidly.Moreover, the compounds in the grain boundaries diffuse evenly, andvariation of strength under high temperature conditions in each portioncan thereby fall within an adequately narrow range.

[0148] A relationship between a zirconium (Zr) content in an alloy, towhich zirconium (Zr) is added as the second component, and grain sizesof the alloy is shown in FIG. 8. In other words, FIG. 8 shows variationof grain sizes in accordance with the amount of zirconium (Zr) added tothe third mode of magnesium alloys according to the present invention,which amount ranges from 0.0 through 1.2 mass %. As shown in FIG. 8, asthe amount of zirconium (Zr) added increases, the grain size of crystalsdecreases. It is shown that when zirconium (Zr) exceeding 0.8 mass % isadded, the effect of addition of zirconium (Zr) shows up at a maximumthereof.

[0149] When the amount of the second component added becomes 1 mass % orgreater, a great number of relatively brittle compounds are generated inthe grains or grain boundaries. Therefore, the relatively brittlecompounds would markedly lower the elongation capability of themagnesium alloys, and lower the strength of the magnesium alloys. It isunderstood that the effect of addition of the second component can beachieved when strontium (Sr) or manganese (Mn) is used, as well.

[0150] Accordingly, the amount of addition of the second componentaccording to the present invention preferably falls below 1 mass %, andmore preferably ranges between 0.5 and 0.8 mass %.

[0151] Consequently, the magnesium alloys composed of the firstcomponent and the second component in addition to magnesium can bringabout inhibition of primary creep deformation due to formation ofsmall-sized grains as well as inhibition of steady-state creep due toformation of eutectic compounds, and can thus achieve effectiveimprovement of the tensile strength and proof stress of magnesium alloysunder high temperature environments.

[0152] As described above, the third mode of magnesium alloys accordingto the present invention exhibits high creep strength under hightemperature conditions, and can thus be employed for structuralmaterials to be used under high temperature conditions, such asstructural materials for a vehicle, in particular, those which lenditself to a cylinder block, a cylinder head, an intake manifold, a headcover, a chain case, an oil pan, a transmission case, an ECU frame, andother structural members to be mounted around the engine of the vehicle.

[0153] The first through third modes of magnesium alloys according tothe present invention as described above may be manufactured for exampleby a casting process as will be described hereafter. That is, themagnesium or magnesium alloys to which a specific amount of yttrium (Y)is added are molten; at a top surface of resulting molten metal of themagnesium or magnesium alloys obtained through the melting step is thenformed a film of oxide made of yttrium (Y), which makes it possible tocast the magnesium alloys according to the present invention, whilepreventing oxidization and combustion of the magnesium or magnesiumalloy.

[0154] Hereupon, the magnesium is composed of magnesium, as a principalingredient, and unavoidable impurities; the magnesium alloy is composedof magnesium, as a principal ingredient, additive elements, andunavoidable impurities. The additive elements are metal elements to beadded to magnesium as appropriate in accordance with intended propertiesof the magnesium alloys. The additive elements that are well knowninclude aluminum, for example.

[0155] The above-described casting process of magnesium alloys accordingto the present invention is designed for such magnesium and magnesiumalloys.

[0156] Hereinafter, such magnesium and magnesium alloys will begenerically called “magnesium materials” as necessary.

[0157] <Molten Magnesium Materials>

[0158] In general, when a part is formed of a magnesium material, firstthe magnesium material is molten into a molten material. The moltenmagnesium material is then cast into a mold, and cooled and solidifiedin the mold, so that the part is formed of the magnesium material.

[0159] In general, features of the molten magnesium material include:upon contact with oxygen in the atmosphere, burning with a dazzlinglight occurs, forming a white powder of magnesium oxide (MgO); and uponcontact with heated iron oxide, vigorous reaction occurs, reducing ironoxide with the liberation of iron as a simple substance, to formmagnesium oxide (MgO). Accordingly, care should be exercised in handlingthe molten magnesium material so as to avoid unintended oxidation andburning, and particularly to avoid contact with oxygen.

[0160] (Yttrium (Y))

[0161] In the casting process of magnesium or magnesium alloys accordingto the present invention, in order to prevent oxidation and burning ofthe molten magnesium material that could be caused by oxygen in theatmosphere during the casting process, yttrium (Y) of which a content is0.002 mass % or greater, preferably 0.002-1.0 mass %, more preferably0.002-0.3 mass % is added to the magnesium material.

[0162] Thereafter, when the magnesium material to which yttrium (Y) isadded gets molten, at a top surface of the molten magnesium material isformed a dense film of oxide including yttrium (Y). The film of oxideserves to keep the molten magnesium material from contact with oxygen inthe atmosphere, and thus prevents oxidation and burning of the moltenmagnesium material.

[0163] It is understood that the minimum amount of addition of yttrium(Y) is the value obtained by experiment. A description will be givenbelow of the reason the amount of addition of yttrium (Y) was determinedwithin the above ranges, with reference to FIG. 9.

[0164]FIG. 9 is a graph showing a relationship between the time requiredtill molten metal of magnesium with an yttrium (Y) content startsburning, and amounts of yttrium (Y) added, where the molten metal ofmagnesium to which yttrium (Y) is added is exposed in anoxygen-containing environment.

[0165] In FIG. 9, when the amount of yttrium (Y) added was less than0.002 mass %, exposure of the molten metal of magnesium, to whichyttrium (Y) was added, to an oxygen-containing environment caused themolten metal of magnesium with an yttrium content to start burningwithin a very short time. This is because a dense film of oxideincluding yttrium (Y) was less formed on a top surface of the moltenmetal of magnesium.

[0166] It has been shown that if the amount of yttrium (Y) added is morethan 0.002 mass %, the effect of inhibition of burning appears andgradually increases with the amount.

[0167] When the amount of yttrium (Y) added were 0.005 mass %, themolten metal of magnesium with the yttrium (Y) content did not burnuntil approximately three minutes had passed after the alloy was putinto an oxygen-containing environment.

[0168] Subsequently, likewise, the time for which the burning were beinginhibited became longer with the increase of the amount of yttrium (Y)added; when the amount of yttrium (Y) added was more than 0.01 mass %,even when the molten metal of magnesium was exposed in anoxygen-containing environment for five minutes, the burning of themolten metal of magnesium did not occur.

[0169] As described above, the experimental results shown in FIG. 9revealed that the lower limit of the amount of yttrium (Y) to be addedto magnesium is 0.002 mass %.

[0170] Consequently, in the casting process by which the magnesium alloyaccording to the present invention may be manufactured, if yttrium (Y)added to a magnesium material is 0.002 mass % or greater, the objects ofthe present invention can be achieved.

[0171] Hereupon, since yttrium (Y) is a relatively expensive element, anincreased amount of yttrium (Y) added would disadvantageously impaircost efficiency. Moreover, an excessive amount of yttrium (Y) addedcould possibly make the yttrium (Y) alloyed with magnesium. For theseand other reasons, the amount of addition of yttrium (Y) in the castingprocess of magnesium materials may preferably be that which may serve toform a film of oxide on a top surface of the molten metal of magnesiummaterials in the minimum amount required to inhibit oxidation andburning of the molten metal of magnesium.

[0172] Accordingly, the upper limit of the amount of yttrium (Y) to beadded to magnesium materials may be appropriately determined inaccordance with the cost of magnesium alloys to be manufactured orworkability of pouring molten metal of magnesium into a mold, and 0.3mass % (producing a film of oxide of approximately 0.05 mm in thickness)is an amount in which the intended effect can be achieved sufficientlywithout impairing the workability.

[0173] According to the casting process of magnesium or magnesium alloysas described above, a film of oxide containing yttrium (Y) is formed ona top surface of the molten metal of the magnesium or magnesium alloys,and thus the molten metal of the magnesium or magnesium alloys is shutout from oxygen contained in the air. Therefore, the oxidation andburning of the molten metal of the magnesium or magnesium alloys can beprevented.

[0174] The effect of addition of yttrium (Y) can be achievedappropriately even when the temperature of the molten metal of magnesiummaterials is high; e.g., 700° C. or higher, and even if the molten metalof magnesium materials to which yttrium (Y) is added is kept under hightemperature conditions for a long time, as were the case with berylliumor calcium used in conventional burning prevention methods, the amountof the element which exists in the molten metal of magnesium materialsis not reduced over time.

[0175] Moreover, when magnesium materials are cast from molten metal ofmagnesium materials to which yttrium (Y) is added, the grain size ofeach crystal structure (particle) of yttrium (Y) does not become coarse.In other words, the grain size of each crystal structure (particle) ofthe magnesium materials resulting from the casting process is restrictedto be small, and thus, the magnesium materials having high heatresistance can be obtained.

[0176] Further, the use of the molten metal of magnesium materials towhich yttrium (Y) is added makes the molten metal of magnesium materialseasy-separable. Thus, when the molten metal of magnesium materials ispoured into the mold, the molten metal of magnesium materials isprevented from clinging to a ladle, unlike molten metal of magnesiumalloys to which no yttrium (Y) is added.

[0177] Furthermore, separation of the mold, when a casting is separatedfrom the mold after the casting is formed, is easier; good workabilitycan thus be achieved.

[0178] These are derived from the peculiar properties of a film of oxidecontaining yttrium (Y), such as a proper wetting characteristics,surface tension, and so forth.

[0179] The present inventors and their colleagues employed the castingprocess of magnesium alloys according to the present invention asdescribed above, and carried out the casting of magnesium alloys inpractice as will be described below.

[0180] In a protective gas (argon (Ar)+5% sulfur hexafluoride (SF₆))atmosphere, pure magnesium to which yttrium (Y) in an amount of 0.028mass % was added, and pure magnesium as a comparative example to whichno yttrium (Y) was added, were respectively heated in a small-sizedsmelting furnace (10 kg) until each melted at 700° C.

[0181] The melting of magnesium was performed in a melting pot made ofboiler steel of which the inside had been calorized. The dimensions ofthe pot were 150 mm in inside diameter, 200 mm in depth, and 177 cm² insurface area of the molten metal in contact with outside air. Thethickness of the film of oxide formed on the surface of the molten metalof magnesium materials was approximately 0.05 mm.

[0182] After the molten metal was kept at 700° C. for five minutes, theprotective gas was stopped, the lid of the pot was removed, and thechange of the surface of the molten metal was observed.

[0183] Even if five minutes have passed since the protective gas wasstopped, the molten metal of the pure magnesium to which yttrium (Y) hadbeen added did not burn.

[0184] On the other hand, the molten metal of pure magnesium to which noyttrium (Y) was added started burning approximately 10 minutes after theprotective gas was stopped, and black granular oxides generated byburning were recognized on the surface of the molten metal of the puremagnesium.

[0185] In order to cast the pure magnesium to which yttrium (Y) wasadded into a boat-shaped mold, molten metal of magnesium was poured intothe mold. During the pouring operation, the molten metal of magnesiumdid not cling to the pot or the ladle, and thus the pouring operationwas performed efficiently.

[0186] As discussed above, the present invention serves to preventmolten metal of magnesium or magnesium alloys from burning by adding apredetermined amount of yttrium (Y) to the magnesium or magnesiumalloys.

[0187] Here, with the melting pot (150 mm in inside diameter, 200 mm indepth, and 177 cm² in surface area of the molten metal in contact withthe outside air) used in the aforementioned example, the thickness ofthe film of oxide formed on the surface of the molten metal of magnesiumwas 0.05 mm when yttrium is added in the amount of 0.3 mass %, and 0.2mm when yttrium is added in the amount of 1.0 mass %.

[0188] More specifically, the larger the amount of yttrium (Y) added,the more stable film of oxide is formed on the surface of molten metalof magnesium. However, it has been shown that if the film of oxidebecomes too thick, the workability in pouring molten metal of magnesiuminto the mold tends to decrease, and that in view of workability, thethickness of the film of oxide should preferably be less than 0.05 mm.

[0189] Accordingly, in order to prevent molten metal of magnesium fromburning as a result of contact with the outside air, the thickness of afilm of oxide to be formed on the surface of molten metal of magnesiumis less than 0.2 mm, and preferably less than 0.05 mm. The lower limitof the thickness of the film of oxide is the thickness of the film ofoxide formed on the surface of molten metal of magnesium to whichyttrium is added in the amount of 0.002 mass %, that is, approximately0.01 mm.

[0190] Alternatively, since the thickness of the film of oxide graduallydecreases when molten metal of magnesium is being poured into the mold,yttrium (Y) may be added as appropriate in accordance with the thicknessof the film of oxide formed on the surface of molten metal of magnesium,or in accordance with the depth/residual amount of the molten metal inthe melting pot.

[0191] The above-described casting process of magnesium alloys isapplied not only to magnesium alloys according to the present invention,but also to magnesium or magnesium alloys in common use.

WORKING EXAMPLES

[0192] A detailed description will hereinafter be given of workingexamples of the present invention. It is however understood that thepresent invention is not limited to these examples.

First Working Examples

[0193] First, to prepare a material having the composition as shown inTable 1, pure magnesium was melted in an atmosphere of a mixture ofgases consisting of argon and sulfur hexafluoride in an electricsmelting furnace, and a first component and a second component each in apredetermined amount were charged, stirred, and rested. The resultingmolten metal was poured into a metal mold 30 mm high, 25 mm wide, and200 mm long, to obtain a casting material.

[0194] The melting process was carried out in a melting pot of which theinside had been calorized, and the charging of the elements wasperformed when the temperature of the pure magnesium was 700° C.

[0195] The casting material was soaked for 100-hr. thermal hysteresis inan atmosphere at 200° C., and thereafter, a tensile test specimen and acreep test specimen were taken out and subjected to the tensile test andthe creep test. JIS No.4 Piece was used for the test specimens.

[0196] The tensile test was conducted using a 5-ton Autograph Tester inan atmosphere at 200° C. at a tensile speed of 0.5 mm/minute. In thecreep test, a load of 50 MPa is given at 200° C. for 100 hrs to measurean entire elongation percentage. TABLE 1 Atomic radius Characteristicsin strength differential at 200° C. between Entire first Eutectic creepelement temperature 0.2% elongation First and Second of second proof 50MPa, element magneium element element stress Elongation 100 Hr (wt %)(%) (wt %) (° C.) (MPa) (%) (%) Comparative 0.5 Ca 23.1  0.8 Si 638 2023.0 Ruptured Example 1 halfway Comparative 2.0 Ca 23.1  1.0 Si 638 4516.5 6.950 Example 2 Comparative 2.0 Al −10.6  0.8 Si 638 70 85 RupturedExample 3 halfway Comparative 4.0 Al −10.6  2.0 Nd 548 100 10.0 2.250Example 4 Comparative 5.0 Al −10.6  3.0 Mm 517 75 6.5 0.520 Example 5Comparative 1.0 Zn −16.9  3.0 Ca 548 90 4.0 0.305 Example 6 Comparative3.5 Gd 11.3 — 87 22.0 1.086 Example 7 Comparative 3.5 Gd 11.3 20.0 Ce634 255 0.2 0.090 Example 8

[0197] In like manner, the results from a test in which gadolinium (Gd)was employed as the first component are shown below in Table 2; theresults from a test in which terbium (Tb) was employed as the firstcomponent are shown below in Table 3; the results from a test in whichdysprosium (Dy) was employed as the first component are shown below inTable 4; the results from a test in which holmium (Ho) was employed asthe first component are shown below in Table 5; and the results from atest in which yttrium (Y) was employed as the first component are shownbelow in Table 6. TABLE 2 Atomic radius Characteristics in strengthdifferential at 200° C. between Eutectic Entire first temper- creepelement ature 0.2% elongation First and Second of second Third proof 50MPa, element magneium element element element stress Elongation 100 Hr(wt %) (%) (wt %) (° C.) (wt %) (MPa) (%) (%) Example 1 0.8 Gd 11.3 9.5Nd 548 — 120 7.5 0.150 Example 2 3.5 Gd 11.3 8.0 Nd 548 — 185 8.0 0.065Example 3 4.1 Gd 11.3 9.7 Nd 548 — 178 3.0 0.102 Example 4 3.4 Gd 11.32.4 Nd 548 — 84 11.5 0.240 Example 5 3.6 Gd 11.3 13.5 Nd 548 — 275 4.20.078 Example 6 3.5 Gd 11.3 8.0 Ce 634 — 175 6.0 0.085 Example 7 3.5 Gd11.3 15.0 Ce 634 — 265 3.0 0.095 Example 8 1.0 Gd 11.3 8.0 Ce 634 — 1207.5 0.150 Example 9 3.5 Gd 11.3 8.0 La 613 — 130 8.0 0.160 Example 103.7 Gd 11.3 7.9 Pr 575 — 163 6.5 0.295 Example 11 3.5 Gd 11.3 6.5 Sm 540— 140 5.4 0.362 Example 12 3.5 Gd 11.3 8.0 MM (540) — 180 6.5 0.075Example 13 3.5 Gd 11.3 8.0 Sn 562 — 130 8.5 0.155 Example 14 3.5 Gd 11.38.0 Ba 634 — 130 2.5 0.115 Example 15 3.7 Gd 11.3 8.5 Nd 548 0.4 Zr 1807.4 0.070 Example 16 3.6 Gd 11.3 8.9 Nd 548 0.8 Zr 187 7.6 0.096 Example17 3.7 Gd 11.3 7.6 Nd 548 1.2 Zr 198 8.5 0.150 Example 18 3.5 Gd 11.38.4 Nd 548 0.5 Sr 185 7.4 0.130 Example 19 3.5 Gd 11.3 8.7 Nd 548 0.7 Mn188 8.3 0.087

[0198] TABLE 3 Atomic radius Characteristics in strength differential at200° C. between Eutectic Entire first temper- creep element ature 0.2%elongation First and Second of second Third proof 50 MPa, elementmagneium element element element stress Elongation 100 Hr (wt %) (%) (wt%) (° C.) (wt %) (MPa) (%) (%) Example 1.2 Tb 10.0 9.0 Nd 548 — 168 8.90.130 20 Example 8.4 Tb 10.0 8.5 Nd 548 — 250 5.5 0.075 21 Example 11.4Tb 10.0 9.0 Nd 548 — 289 3.9 0.065 22 Example 8.6 Tb 10.0 14.0 Nd 548 —315 2.5 0.110 23 Example 8.2 Tb 10.0 6.4 Ce 634 — 250 6.5 0.100 24Example 5.0 Tb 10.0 8.0 MM (540) — 205 5.0 0.150 25 Example 7.9 Tb 10.07.0 Sn 562 — 180 4.5 0.156 26 Example 8.1 Tb 10.0 8.9 Nd 548 0.7 Zr 2505.8 0.120 27 Example 7.3 Tb 10.0 8.4 Nd 548 0.3 Sr 260 6.6 0.120 28Example 8.2 Tb 10.0 8.7 Nd 548 0.6 Mn 260 4.5 0.115 29

[0199] TABLE 4 Atomic radius Characteristics in strength differential at200° C. between Eutectic Entire first temper- creep element ature 0.2%elongation First and Second of second Third proof 50 MPa, elementmagneium element element element stress Elongation 100 Hr (wt %) (%) (wt%) (° C.) (wt %) (MPa) (%) (%) Example 0.6 Dy 9.4 9.5 Nd 548 — 144 9.50.150 30 Example 5.0 Dy 9.4 8.0 Nd 548 — 215 7.0 0.065 31 Example 11.4Dy 9.4 9.7 Nd 548 — 305 6.0 0.085 32 Example 9.8 Dy 9.4 2.4 Nd 548 — 2206.8 0.159 33 Example 9.5 Dy 9.4 14.1 Nd 548 — 285 3.5 0.065 34 Example8.9 Dy 9.4 5.9 Ce 634 — 235 4.0 0.125 35 Example 9.7 Dy 9.4 8.0 MM (540)— 195 5.5 0.120 36 Example 9.4 Dy 9.4 6.5 Sm 540 — 200 5.5 0.150 37Example 9.4 Dy 9.4 7.5 Sn 562 — 195 4.5 0.097 38 Example 9.6 Dy 9.4 7.2Ba 634 — 193 3.8 0.105 39 Example 8.9 Dy 9.4 8.9 Nd 548 0.6 Zr 210 7.50.078 40 Example 9.9 Dy 9.4 7.6 Nd 548 1.2 Zr 237 8.9 0.085 41 Example9.4 Dy 9.4 8.4 Nd 548 0.5 Sr 218 8.0 0.078 42 Example 9.3 Dy 9.4 8.7 Nd548 0.5 Mn 210 7.6 0.080 43

[0200] TABLE 5 Atomic radius Characteristics in strength differential at200° C. between Eutectic Entire first temper- creep element ature 0.2%elongation First and Second of second Third proof 50 MPa, elementmagneium element element element stress Elongation 100 Hr (wt %) (%) (wt%) (° C.) (wt %) (MPa) (%) (%) Example 0.6 Ho 10.0 9.0 Nd 548 — 200 8.00.150 44 Example 8.4 Ho 10.0 9.2 Nd 548 — 285 6.5 0.064 45 Example 11.4Ho 10.0 9.7 Nd 548 — 270 4.0 0.050 46 Example 8.6 Ho 10.0 13.5 Nd 548 —298 2.0 0.136 47 Example 8.2 Ho 10.0 6.5 Ce 634 — 240 4.0 0.150 48Example 5.0 Ho 10.0 8.0 MM (540) — 205 5.0 0.150 49 Example 8.1 Ho 10.06.5 Sm 540 — 240 4.2 0.170 50 Example 7.9 Ho 10.0 7.5 Sn 562 — 180 8.90.132 51 Example 8.3 Ho 10.0 8.8 Nd 549 0.8 Zr 275 6.0 0.120 52 Example8.5 Ho 10.0 8.1 Nd 551 0.5 Sr 270 5.0 0.090 53 Example 8.6 Ho 10.0 8.1Nd 552 0.6 Mn 270 5.5 0.120 54

[0201] TABLE 6 Atomic radius Characteristics in strength differential at200° C. between Eutectic Entire first temper- creep element ature 0.2%elongation First and Second of second Third proof 50 MPa, elementmagneium element element element stress Elongation 100 Hr (wt %) (%) (wt%) (° C.) (wt %) (MPa) (%) (%) Example 55 0.4 Y 13.8 9.0 Nd 548 — 10016.5 0.165 Example 56 2.0 Y 13.8 8.2 Nd 548 — 160 8.0 0.100 Example 575.0 Y 13.8 8.0 Nd 548 — 140 5.5 0.130 Example 58 2.0 Y 13.8 14.5 Nd 548— 220 4.3 0.145 Example 59 1.8 Y 13.8 8.3 Ce 634 — 165 5.0 0.095 Example60 1.8 Y 13.8 7.0 MM (540) — 160 6.5 0.190 Example 61 1.9 Y 13.8 6.8 Sm540 — 135 11.0 0.140 Example 62 2.0 Y 13.8 5.0 Sn 562 — 145 15.5 0.180Example 63 2.0 Y 13.8 8.0 Nd 549 0.4 Zr 170 8.5 0.135 Example 64 2.0 Y13.8 8.5 Nd 551 0.2 Sr 165 8.5 0.130 Example 65 1.9 Y 13.8 5.4 Nd 5520.5 Mn 165 7.0 0.160

[0202] As a result, the magnesium alloys prepared by adding the first,second and third components that each meet the specific conditions(Examples 1 through 65) indicated more advantageous properties than theexamples (Comparative examples 1 and 2) where calcium of which theatomic radius is different from that of magnesium by 14% or more wasemployed, and the examples (Comparative examples 3 through 6) wherealuminum, zinc, or the like each having a radius shorter than magnesiumatom has. Consequently, the utilities of the magnesium alloys accordingto the present invention were assured.

Second Working Examples

[0203] Next, to prepare a material having the composition as shown inTable 7, pure magnesium was melted in an atmosphere of a mixture ofgases consisting of argon and sulfur hexafluoride in an electricsmelting furnace, and a first component and a second component each in apredetermined amount were charged, stirred, and rested. The resultingmolten metal was poured into a metal mold 30 mm high, 25 mm wide, and200 mm long, to obtain a casting material.

[0204] Misch metal (MM) was used as elements among lanthanoids, withinthe range from lanthanum through europium.

[0205] Moreover, the melting process was carried out in a melting pot ofwhich the inside had been calorized, and the charging of the elementswas performed when the temperature of the pure magnesium was 700° C.

[0206] The casting material was soaked for 100-hr. thermal hysteresis inan atmosphere at 200° C., and thereafter, a tensile test specimen and acreep test specimen were taken out and subjected to the tensile test andthe creep test (both in the form of JIS No.4 Piece).

[0207] The tensile test was conducted using a 5-ton Autograph Tester inan atmosphere at 200° C. at a tensile speed of 0.5 mm/minute. In thecreep test, a load of 50 MPa is given at 200° C. for 100 hrs to measurean entire elongation percentage. The results are shown in FIG. 7. TABLE7 Characteristics in strength at 200° C. 0.2% Entire creep First SecondThird proof Elongation elongation (%) element element element stress onrupture 50 MPa, (mass %) (mass %) (mass %) (MPa) (%) 100 Hr Example 660.8 Gd 9.5 Nd — 120 7.5 0.150 Example 67 3.5 Gd 8.0 Nd — 185 8.0 0.065Example 68 4.1 Gd 9.7 Nd — 178 3.0 0.102 Example 69 3.4 Gd 2.4 Nd — 8411.5 0.240 Example 70 3.6 Gd 13.5 Nd — 275 4.2 0.078 Example 71 3.5 Gd8.0 Ce — 175 6.0 0.085 Example 72 3.5 Gd 15.0 Ce — 265 3.0 0.095 Example73 1.0 Gd 8.0 Ce — 120 7.5 0.150 Example 74 3.5 Gd 8.0 La — 130 8.00.160 Example 75 3.7 Gd 7.9 Pr — 163 6.5 0.295 Example 76 3.5 Gd 6.5 Sm— 140 5.4 0.362 Example 77 3.5 Gd 8.0 MM — 180 6.5 0.075 Example 78 3.7Gd 8.5 Nd 0.4 Zr 180 7.4 0.070 Example 79 3.6 Gd 8.9 Nd 0.8 Zr 187 7.60.096 Example 80 3.7 Gd 7.6 Nd 1.2 Zr 198 8.5 0.150 Example 81 3.5 Gd8.4 Nd 0.5 Sr 185 7.4 0.130 Example 82 3.5 Gd 8.7 Nd 0.7 Mn 188 8.30.087 Comparative 3.5 Gd — — 87 22.0 1.086 example 9 Comparative 3.5 Gd20.0 Ce — 255 0.2 0.090 example 10 Comparative 0.5 Ca 0.8 Si — 20 23.0Ruptured example 1 halfway Comparative 2.0 Ca 1.0 Si — 45 16.5 6.950example 12 Comparative 2.0 AL 0.8 Si — 70 8.5 Ruptured example 13halfway Comparative 4.0 AL 2.0 Nd — 100 10.0 2.250 example 14Comparative 5.0 AL 3.0 MM — 50 13.0 1.730 example 15 Comparative 5.0 AL3.0 Ca — 75 6.5 0.520 example 16 Comparative 1.0 Zn 3.0 Nd — 90 4.00.305 example 17

[0208] As can be seen from Table 7, the magnesium alloys prepared byadding the first, second and, if desired, third components that eachmeet the specific conditions (kinds of elements and amounts thereof)required by the present invention (Examples 66 through 82) indicatedmore advantageous properties than the examples (Comparative examples 9and 10) where cerium (Ce) as the second component less than 1 mass % ormore than 15 mass % was employed, and the conventional magnesium alloys(Comparative examples 11 through 17). Consequently, excellent propertiesat high temperatures of the magnesium alloys according to the presentinvention were assured.

Third Working Examples

[0209] Next, to prepare a material having the composition as shown inTable 8, pure magnesium was melted in an atmosphere of a mixture ofgases consisting of argon and sulfur hexafluoride in an electricsmelting furnace, and a first component and a second component each in apredetermined amount were charged, stirred, and rested. The resultingmolten metal was poured into a metal mold 30 mm high, 25 mm wide, and200 mm long, to obtain a casting material.

[0210] The melting process was carried out in a melting pot of which theinside had been calorized, and the charging of the elements wasperformed when the temperature of the pure magnesium was 700° C.

[0211] The casting material was soaked for 100-hr. thermal hysteresis inan atmosphere at 150° C., and thereafter, a tensile test specimen and acreep test specimen were taken out and subjected to the tensile test andthe creep test (both in the form of JIS No.4 Piece).

[0212] The tensile test was conducted using a 5-ton Autograph Tester inan atmosphere at 150° C. at a tensile speed of 0.5 mm/minute. In thecreep test, a load of 50 MPa is given at 150° C. for 100 hrs to measurean entire elongation percentage. TABLE 8 Characteristics in creep at200° C. Entire Creep Compostion of alloy Cerium/Tin elongation (%) (Unitis mass %) ratio 50 MPa 100 Hr Example 83 Mg-5.0Ce-3.6Sn 1.4 0.234Example 84 Mg-5.0Ce-4.5Sn 1.1 0.193 Example 85 Mg-4.5Ce-6.4Sn 0.7 0.245Example 86 Mg-8.0Ce-6.2Sn 1.3 0.136 Example 87 Mg-4.0Ce-4.0Sn 1.0 0.161Example 88 Mg-4.0Ce-6.7Sn 0.6 0.185 Example 89 Mg-6.0Ce-5.0Sn 1.2 0.179Comparative Mg-0.5Ce-1.0Sn 0.5 Ruptured halfway example 18 ComparativeMg-1.0Ce-5.0Sn 0.2 Ruptured halfway example 19 ComparativeMg-5.0Ce-3.3Sn 1.5 0.550 example 20 Comparative Mg-14.0Ce-7.0Sn 2.00.358 example 21 Comparative Mg-0.5Ca-1.0Si — Ruptured halfway example22 Comparative Mg-2.0Ca-1.0Si — 6.950 example 23 ComparativeMg-2.0A1-0.8Si — Ruptured halfway example 24 Comparative Mg-4.0A1-2.0Nd— 2.250 example 25 Comparative Mg-5.0A1-3.0MM — 1.730 example 26Comparative Mg-3.0Nd-1.0Zn — 0.305 example 27 Comparative Mg-5.0A1-3.0Ca— 0.520 example 28 Comparative Mg-5.0A1-2.0Ca-2.0MM — 0.755 example 29

[0213] As a result, it has been shown that if the amounts of addition ofcerium (Ce) and tin (Sn) in the resulting magnesium alloys each fallwithin a predetermined range, and if a ratio of addition of cerium (Ce)and tin (Sn) in mass (cerium/tin ratio or Ce/Sn ratio) falls within arange of 0.6 to 1.4 (Examples 83 through 89), the entire creepelongation (%) can be restricted, in comparison with the alloys havingconventional compositions (Comparative examples 29 through 36) or thelike.

[0214] Moreover, when the amount of addition of cerium (Ce) and tin (Sn)were both less than the lower limit (2 mass % and 10 mass %respectively) (Comparative example 25), or when the amount of additionof cerium (Ce) only was less than the lower limit (2 mass %)(Comparative example 26), the magnesium alloys were ruptured halfwayduring the creep test. It is presumably because the amount of eutecticcompounds formed was too small to inhibit deformation of the magnesiumalloys.

[0215] Further, when the amounts of addition of cerium (Ce) and tin (Sn)each fell within a predetermined range, and if the ratio of addition ofcerium (Ce) and tin (Sn) in mass (cerium/tin ratio or Ce/Sn ratio) wasout of a range of 0.6 to 1.4 (Example 3), the entire creep elongations(%) were substantially equal to or better than those of the alloyshaving conventional compositions (Comparative examples 29 through 36).

[0216] Therefore, it has been shown that the creep elongation (%) of themagnesium alloys to which cerium (Ce) and tin (Sn) each in apredetermined range of amounts are added shows a better result thanthose of the magnesium alloys having conventional compositions, and moredesirable result can be achieved if the ratio of cerium (Ce) and tin(Sn) to be added falls within a range from 0.6 to 1.4.

[0217] Consequently, the utilities of the magnesium alloys according tothe present invention were assured.

[0218] It is understood that the present invention is not limited to theaforementioned forms of the present invention, but that other forms asfollows for example fall within the scope of the present invention.

[0219] (First Variation)

[0220] (1) A refractory magnesium alloy, which includes magnesium as aprincipal ingredient, and an element having a radius 9-14% larger than amagnesium atom and a maximum concentration of 2 mass % or larger in asolid solution with magnesium, which element is mixed in an amount notexceeding a maximum amount that can be homogeneously mixed in the solidsolution with magnesium, whereby internal strength of grains thereof isenhanced.

[0221] (2) A refractory magnesium alloy, which further includes anelement having an eutectic temperature of 540° C. with magnesium, whichelement with a content thereof ranging from 1 to 15 mass % is added.

[0222] (3) A structural material for a vehicle which is made up of theabove refractory magnesium alloys.

[0223] (Second Variation)

[0224] (1) A refractory magnesium alloy, which includes magnesium as aprincipal ingredient, and gadolinium with a content thereof ranging from0.5 to 3.8 mass %, wherein remaining part other than the gadolinium iscomposed of the magnesium and unavoidable impurities.

[0225] (2) A refractory magnesium alloy, in addition to the aboverefractory magnesium alloy, further includes at least one elementselected from a group consisting of lanthanum through europium amonglanthanoids in the periodic table of the elements with a content thereofranging from 1 to 15 mass %.

[0226] (3) A refractory magnesium alloy, in addition to the aboverefractory magnesium alloy, further includes at least one elementselected from a group consisting of zirconium, strontium and manganesewith a content thereof falling below 1 mass %.

[0227] (4) A structural material for a vehicle which is made up of theabove refractory magnesium alloys.

[0228] (Third Variation)

[0229] (1) A refractory magnesium alloy, which includes magnesium as aprincipal ingredient, and gadolinium with a content thereof ranging from0.5 to 3.8 mass %, wherein remaining part other than the gadolinium iscomposed of the magnesium and unavoidable impurities.

[0230] (2) A refractory magnesium alloy, in addition to the aboverefractory magnesium alloy, further includes at least one elementselected from a group consisting of lanthanum through europium amonglanthanoids in the periodic table of the elements with a content thereofranging from 1 to 15 mass %.

[0231] (3) A refractory magnesium alloy, in addition to the aboverefractory magnesium alloy, further includes at least one elementselected from a group consisting of zirconium, strontium and manganesewith a content thereof falling below 1 mass %.

[0232] (4) A structural material for a vehicle which is made up of theabove refractory magnesium alloys.

INDUSTRIAL APPLICABILITY

[0233] The present invention realizes magnesium alloys that have both ofhigh proof stress and high creep strength at high temperatures, and thusmay be employed for reinforcing members to be exposed to hightemperature conditions such as engines for a vehicle, so that decreasein axial force in a bolt-fastened portion can be restricted to theminimum, and a significant reduction in weight of a vehicle body can beachieved.

[0234] Moreover, the present invention realizes magnesium alloys thathave high creep strength at 150° C. or higher, and thus may be employedfor reinforcing members to be exposed to high temperature conditionssuch as engines for a vehicle, so that decrease in axial force in abolt-fastened portion can be restricted to the minimum, and asignificant reduction in weight of a vehicle body can be achieved.

[0235] Further, the present invention realizes a casting process ofmagnesium or magnesium alloys by which magnesium or magnesium alloys maybe cast without allowing molten metal of magnesium or magnesium alloysto be oxidized or burnt in the presence of oxygen, and thus makes iteasier to cast the magnesium or magnesium alloys than a conventionalmethod of casting magnesium or magnesium alloys while preventing themolten metal of the magnesium or magnesium alloys from being oxidized orburnt.

1. A magnesium alloy, which includes magnesium as a principal ingredient, and an element having a radius 9-14% larger than a magnesium atom and a maximum concentration of 2 mass % or larger in a solid solution with magnesium, wherein the element is mixed in an amount not exceeding a maximum amount that can be homogeneously mixed in the solid solution with magnesium, whereby internal strength of grains thereof is enhanced.
 2. A magnesium alloy according to claim 1, which further includes an element having an eutectic temperature of 540° C. with magnesium, wherein the element with a content thereof ranging from 1 to 15 mass % is added.
 3. A structural material for a vehicle which is made up of the magnesium alloy according to claim
 1. 4. A structural material for a vehicle which is made up of the magnesium alloy according to claim
 2. 5. A magnesium alloy, which includes magnesium as a principal ingredient, and gadolinium with a content thereof ranging from 0.5 to 3.8 mass %, wherein remaining part other than the gadolinium is composed of the magnesium and unavoidable impurities.
 6. A magnesium alloy according to claim 5, further includes at least one element selected from a group consisting of lanthanum through europium among lanthanoids in the periodic table of the elements with a content thereof ranging from 1 to 15 mass %.
 7. A magnesium alloy according to claim 5, further includes at least one element selected from a group consisting of zirconium, strontium and manganese with a content thereof falling below 1 mass %.
 8. A magnesium alloy according to claim 6, further includes at least one element selected from a group consisting of zirconium, strontium and manganese with a content thereof falling below 1 mass %.
 9. A structural material for a vehicle which is made up of the magnesium alloy according to claim
 5. 10. A structural material for a vehicle which is made up of the magnesium alloy according to claim
 6. 11. A structural material for a vehicle which is made up of the magnesium alloy according to claim
 7. 12. A structural material for a vehicle which is made up of the magnesium alloy according to claim
 8. 13. A magnesium alloy, which includes magnesium as a principal ingredient, and cerium with a content thereof ranging from 2.0 to 10.0 mass %, tin with a content thereof ranging from 1.4 to 7.0 mass %, wherein remaining part other than the cerium and the tin is composed of the magnesium and unavoidable impurities.
 14. A magnesium alloy according to claim 13, wherein a ratio of cerium to tin (cerium/tin) falls within a range from 0.6 to 1.4.
 15. A magnesium alloy according to claim 13, further includes at least one element selected from a group consisting of zirconium, strontium and manganese with a content thereof falling below 1 mass %.
 16. A magnesium alloy according to claim 14, further includes at least one element selected from a group consisting of zirconium, strontium and manganese with a content thereof falling below 1 mass %.
 17. A structural material for a vehicle which is made up of the magnesium alloy according to claim
 13. 18. A structural material for a vehicle which is made up of the magnesium alloy according to claim
 14. 19. A structural material for a vehicle which is made up of the magnesium alloy according to claim
 15. 20. A structural material for a vehicle which is made up of the magnesium alloy according to claim
 16. 